Why can't Xcode find SDL_mixer?

Hi all, I'm having a lot of problems with SDL_mixer...
My project is set up as "SDL OpenGL Application", and I include the following frameworks:
Linked: Cocoa, OpenGL
Other: SDL, SDL_mixer, Foundation, AppKit
When I try to build my project Xcode says "SDL_mixer.h: No such file or directory".

I've reinstalled SDL_mixer (tried both the runtime and developer versions...I didn't trust my inexperienced self to build it from source) countless times, choosing different "Frameworks" folders because I thought it might make some difference. I made sure, though, to have only one copy of the framework on my drive at once.

The first time I tried to add SDL_mixer, when I selected it in the Groups & Files pane, it displayed only its toolbox icon, with no headers inside it. When I checked in the Finder, there was no SDL_mixer.h file, just an empty framework.

I reinstalled from the developer version, and when I looked inside the new one it had the header there. So far, so good, I thought, but when I tried building again (after cleaning the target / trashing the build folder), Xcode still couldn't find SDL_mixer.h. Tried a new project. Still no luck.

This is so aggravating. I moved my app from GLUT to SDL in about half an hour last night, but this struggle with SDL_mixer alone has consumed some seven hours now. Many thanks for any help.

Go to my site (link below) and in the download section you'll find my (almost) entry for Vectorized. Download the one that says "Src". It uses SDL & SDL_mixer, so you can open the included Xcode project and check to see if you have anything configured different.
WARNING: I may have a lot of uneeded stuff enabled on it, just check for anything that says SDL_mixer

PowerMacX- I like your game. My project is vector-based too and also missed Vectorized (doesn't matter, the point of the contest was to get people to make something and we did)

So after messing with some more settings, I finally tried changing the include from:
#include "SDL_mixer.h" to
#include "SDL_mixer/SDL_mixer.h", and this finally gave me good results. But I sure don't understand why...